The invention concerns a circuit arrangement for operating a high pressure discharge lamp comprising:
input terminals for connection to a supply voltage source; PA1 lamp connection terminals for connection of the lamp, PA1 power supply means coupled to the input terminals for supplying a current to the lamp consisting of successive periods of current and comprising control means suitable for generating a current pulse superpositioned on periods of the current to the lamp.
Such a circuit arrangement is known from U.S. Pat. No. 5,608,294. The known circuit arrangement provides a measure to suppress flickering of a high pressure discharge lamp and is in particular suitable for operating a high pressure discharge lamp in a projection system like a projection television apparatus. In the known circuit arrangement, the lamp is supplied with a current of successive block shaped periods of opposite polarity. The suppression of flickering is reached by supplying to periods of the lamp current additional current pulses with the same polarity at the end of a predetermined fraction of such a period of a lamp current. By means of the thus reshaped current period, the temperature of the electrode is raised to a relatively high value, which high temperature increases the stability of the discharge arc, because the discharge arc originates from the same place on the electrode in each cathodic phase and so flickering is substantially suppressed. The additional current is supplied in a regular sequence, preferably at each successive period. This flickering suppression is of utmost importance as any instability in the discharge position in translated in an optical system like a projection system into a annoying flickering on the projection screen.
In circumstances where the operated lamp forms a part of a projection system with a liquid crystal display (further called LCD display), it has occurred that irregular brightness fluctuations can appear in the projected image. These are a nuisance for the spectator, in particular when those fluctuations have such rate of appearance that they gave the impression of a moving spot of different brightness.